The invention relates to an electromagnetic deflection unit for a cathode ray tube, comprising a hollow coil holder which supports a set of coils for electron beam deflection, said coils having parts extending substantially in the longitudinal direction of the coil holder at its inner side, and field correction elements at the re-entrant side of said coils. The invention also relates to a colour display tube provided with such a deflection unit.
Such a deflection unit and a cathode ray tube (particularly a colour display tube) provided with such a unit are commercially available.
The increasingly stringent requirements which are imposed on display systems with cathode ray tubes and deflection coils lead to improved designs. However, errors always remain. These may be linearity errors, raster distortion errors (in monochrome display tubes) or convergence errors (in colour display tubes). Some of these errors are produced during manufacture due to spreading of the manufacturing process. When the tube and the coil are assembled, it is attempted in practice to correct a part of the errors which arises during manufacture by adding field correction elements which are generally in the form of preformed elements sometimes referred to as "spoilers". These elements may be made of magnetic material or of electrically conducting material (for example Al or Cu). The addition is realized locally with reference to errors found by an operator. This results in a local improvement of quality. To be able to generate a maximum possible field strength at a minimum possible power, the coils are arranged at the inner side of the coil holder in such a way that--when the deflection unit is mounted on a display tube--the coils are located as close as possible to the electron beams in the display tube. As a result of this deflection unit structure, the greater part of the preformed elements must generally be glued against the inner surface of the coils. The rather non-flat surface of the coils may bring about a poor adhesion so that the preformed elements come loose. It is conventional practice to secure soft-magnetic preformed elements by means of adhesive tape and fix them with glue.
The (manual) correct positioning and fixation of the preformed elements, which may have mutually different shapes and sizes, is time-consuming and hence expensive so that generally a relatively small number, for example, four or six is used. Generally this results only in a local improvement of the picture displayed on the screen. The results of this known method of optimization are therefore inadequate if more stringent requirements are imposed.